Hydrothermal Synthesis of Pt-, Fe-, and Zn-doped SnO 2 Nanospheres and Carbon Monoxide Sensing Properties

Pure and M-doped (M=Pt, Fe, and Zn) SnO 2 nanospheres were successfully synthesized via a simple and facile hydrothermal method and characterized by X-ray powder diffraction, field-emission scanning electron microscopy, and energy dispersive spectroscopy. Chemical gas sensors were fabricated based on the as-synthesized nanostructures, and carbon monoxide sensing properties were systematically measured. Compared to pure, Fe-, and Zn-doped SnO 2 nanospheres, the Pt-doped SnO 2 nanospheres sensor exhibits higher sensitivity, lower operating temperature, more rapid response and recovery, better stability, and excellent selectivity. In addition, a theoretical study based on the first principles calculation was conducted. All results demonstrate the potential of Pt dopant for improving the gas sensing properties of SnO 2 -based sensors to carbon monoxide.


Introduction
Carbon monoxide (CO), mainly generated from the burning of fossil fuels and malfunctioning equipments, is a kind of colorless, tasteless, widespread, and toxic gas [1][2][3][4][5].Simultaneously, CO is an important fault characteristic gas dissolved in power transformer oil and has been widely used in evaluating the insulation performance of running transformer [6][7][8].Therefore, it is essential to develop high performance CO gas sensors for environmental monitoring and industrial applications [9][10][11][12][13].
Due to the advantages of simple fabrication process, low maintenance cost, and long useful life, metal oxide semiconductors, such as SnO 2 [14], ZnO [15], In 2 O 3 [16], TiO 2 [17], WO 3 [18], and NiO [19], have been widely used for fabricating chemical gas sensors.Among these oxide sensors, SnO 2 , as an interesting chemically and thermally stable n-type semiconductor with wide band gap energy and large exciton binding energy, is regarded as one of the most promising sensing materials against CO gas, and the development of SnO 2 sensors for CO detection has been the focus of numerous research works in the world [20][21][22][23][24].However, there still exist some limitations needed to be further improved, such as low sensitivity, poor selectivity, and bad stability.
At present, numerous efforts have been made to improve the sensing properties of metal oxide semiconductor sensors, among which, the strategy of doping modification with various metallic elements, for example, noble metal [25,26], rare-earth metal [27], transition metal [28], and metal oxide [29], has been proven effective.Due to unique catalytic and electronic activities, these dopants are highly commended for catalyzing various gas sensing reactions.The current reports on the sensing behaviors of various metals-doped SnO 2 are mainly based on trial-and-error experiments, and its gas sensing mechanism remains controversial.Till now, the first principles theoretical calculation has been successfully used to investigate structural [30], electronic [31], magnetism [32], optical [33], and gas sensing properties [34,35] of semiconductor SnO 2 .
To the best of our knowledge, reports on the synthesis of Pt, Fe and Zn-doped SnO 2 nanospheres and their sensing properties against CO are rare, and there is no literature about an atomic level understanding of its sensing mechanism.For these reasons, an atomic level understanding of CO gas sensing behaviors of Pt-, Fe-, and Zn-doped SnO 2 is performed.
Herein, we present a simple, facile, and green hydrothermal synthesis of pure and M-doped (M = Pt-, Fe-, and Zn) SnO 2 nanospheres.To demonstrate the potential applications, chemical gas sensors were fabricated with the as-synthesized SnO 2 nanospheres, and CO gas sensing properties were investigated in detail.The Pt-doped SnO 2 nanospheres sensor exhibits significantly enhanced CO sensing performances in terms of high sensitivity, low operating temperature, better selectivity, rapid response and recovery time, and good reproducibility in comparison with those of pure, Fe-, and Zn-doped SnO 2 nanospheres.In addition, with the first principles, a theoretical calculation was performed to further understand its sensing mechanism.

Synthesis and Characterization of SnO 2 Nanoparticles.
Pt-, Fe-, and Zn-doped SnO 2 nanospheres were prepared by a simple and facile hydrothermal synthesis route [14].All the raw chemicals are analytical-grade reagents purchased from Beijing Chemicals Co. Ltd. and used as received without any further purification.
The detailed synthesis process of Pt-doped SnO 2 nanospheres is as follows.Typically, 20 mL of absolute ethanol, 20 mL of distilled water, 5.0 mmol SnCl 4 ⋅ 5H 2 O, 0.10 mmol H 2 PtCl 6 ⋅ 6H 2 O, and 30 mmol ammonia hydroxide were mixed together in a 100 mL capacity beaker and magnetically stirred at room temperature for 30 min.Then, the fully mixed precursor was transferred into a 100 mL Teflon-lined stainless steel autoclave, which was completely sealed and subsequently heated at 200 ∘ C for 24 h in an electric furnace to produce the precipitate.After cooling to room temperature, the prepared precipitate was harvested by centrifugation and washed with distilled water and absolute ethanol three times, respectively.Finally, the product was dried at 80 ∘ C in air for 24 h.
Fe-and Zn-doped SnO 2 nanospheres were obtained in the same process except that 0.10 mmol FeCl 3 ⋅ 6H 2 O and Zn(CH 3 COO) 2 ⋅ 2H 2 O were added as the source of Fe 3+ or Zn 2+ ions, respectively.In this study, pure SnO 2 nanospheres were also synthesized via hydrothermal method for the purpose of comparison.
The crystalline structures of the as-prepared pure, Pt-, Fe-, and Zn-doped SnO 2 nanospheres were performed by X-ray powder diffraction (XRD) at room temperature using a Rigaku D/max-1200X diffractometer (Tokyo, Japan) with Cu K 1 radiation (40 kV, 200 mA, and  = 1.5406Å).A Nova 400 Nano Field-emission scanning electron microscopy (FESEM, FEI, Hillsboro, OR, USA) equipped with an Oxford Inca 250 EDS energy-dispersive spectroscopy detector was used to characterize the surface morphologies and chemical compositions of the prepared samples.

Fabrication and Measurement of SnO 2 -Based Sensors.
Chemical gas sensors based on the as-synthesized SnO 2 nanospheres were fabricated as follows.First, the assynthesized samples were mixed with diethanolamine and absolute ethanol at a weight ratio of 8 : 1 : 1 to form a paste.It was subsequently screen printed onto an alumina ceramic tube, where a pair of Au electrodes has been previously placed at each side.And then, the alumina ceramic tube was dried at 100 ∘ C for 2 h in air and sintered at 400 ∘ C for 2 h.Finally, an Ni-Cr heating wire was inserted into the tube to form a side-heated gas sensor.The structure diagram of the sensor is similar to that presented in our previous works [7,14].The as-prepared sensor was further aged at 200 ∘ C for a week in an aging test chamber to improve its stability and repeatability.
The gas sensing properties were measured by a CGS-8 intelligent gas sensing analysis system purchased from Beijing Elite Tech Co., Ltd., China.The operating temperature of the sensor could be adjusted precisely through varying the heating current of the Ni-Cr heater.When a smooth and stable baseline resistance was reached, a certain concentration of CO gas was injected into the test chamber.All tests were performed at constant environment temperature and relative humidity.Gas response in this study was defined as  =   /  , where   and   are the sensor resistance in air and in target gas, respectively [26].The time taken by the sensor to achieve 90% of the total resistance change was designated as the response time in the case of gas adsorption or the recovery time in the case of gas desorption [4].

Computational Method.
Based on the density functional theory (DFT), the first principles calculations were performed using the CASTEP code (Cambridge Sequential Total Energy Package) in this study [36][37][38][39][40]. Generalized gradient approximation (GGA) in the scheme of revised-Perdew-Burke-Ernzerhof (RPBE) was employed to describe the exchange and correlation functional between electrons.Interaction between nuclei and electrons was approximated with planewave ultrasoft pseudopotential [7,34].The valence electron configurations for C, O, and Sn atoms were chosen as 2s 2 2p 2 , 2s 2 2p 4 , and 5s 2 5p 2 , respectively, for structural and electronic calculations.The cut-off energy of the planewave was set at 380 eV throughout this work, which ensures a maximum root-mean-square convergent tolerance of 1.0 × 10 −6 eV/atom.In the whole process of geometry optimization and energy calculation, atoms were freely allowed to relax in all directions.

Results and Discussion
3.1.Structural Characterization.X-ray powder diffraction measurement was firstly performed to check the crystalline structures of the synthesized nanostructures.Figure 1 shows the typical XRD patterns of the prepared pure, Pt-, Fe-, and Zn-doped SnO 2 samples.It can be clearly seen in Figure 1 that the synthesized samples are polycrystalline in nature and the XRD patterns of the prepared pure and M-doped SnO  rutile SnO 2 phases, and due to the low amount of dopants (2 at%) in the samples, there is no indication of the presence of other metal oxide diffraction peaks, implying a high purity of our products.
An energy dispersive spectroscopy analysis was utilized to confirm the chemical compositions of the synthesized samples.Figure 2 shows the EDS spectra of pure and Mdoped SnO 2 nanospheres.As shown in Figure 2(a), only Sn, O, C, and Cu peaks are observed for pure SnO 2 , and no other peaks have been found.Nevertheless, characteristic peaks from Fe, Pt, and Zn elements are measured in Figures 2(b), 2(c), and 2(d), respectively.Simultaneously, the radius of Pt 4+ (0.625 Å), Fe 3+ (0.645 Å), and Zn 2+ (0.72 Å) is obviously less than or nearly equal to that of Sn 4+ (0.71 Å); so, it is believed Pt 4+ , Fe 3+ , and Zn 2+ could be doped into the SnO 2 matrix.Thus, based on the EDS results, we believe that Pt 4+ , Fe 3+ , and Zn 2+ ions have been successfully incorporated into the SnO 2 nanostructures [26].
Field-emission scanning electron microscopy was further used to research the surface structural characteristics of the prepared samples.Typical FESEM micrographs are recorded and shown in Figure 3.As seen in Figures 3(a)-3(d), beautiful nanospheres are observed for pure and M-doped SnO 2 nanostructures.These nanospheres are rather dispersed and highly uniform in shape and size.The diameters of the samples are ranging from 35 to 40 nm.On comparison of the morphology of pure SnO 2 with that of the M-doped samples, similar microstructure and surface morphology are observed, which indicates that the dopant of Pt 2+ , Fe 3+ , and Zn 2+ ions affects the microstructure and morphology of SnO 2 nanostructures slightly.

Gas Sensing Properties.
To investigate how the ions doped in SnO 2 nanospheres influence the gas sensing properties of the sensors, we first measured the gas responses of the pure and Pt-, Fe-, and Zn-doped sensors to 100 ppm of CO at a series of operating temperatures ranging from 100 ∘ C to 450 ∘ C. As shown in Figure 4 for each sensor, the gas response curve increases and arrives to its maximum at a certain temperature, namely, the optimum operating temperature, and then decreases rapidly with further increasing operating temperature.The optimum operating temperatures of the Pt-, Fe-, and Zn-doped SnO 2 sensors are measured to be about 300, 350, and 350 ∘ C, respectively.It is also observed that at a same working temperature, the Pt-doped SnO 2 sensor exhibits obviously higher gas response than that of pure, Fe-, and Zn-doped SnO 2 sensors.The highest gas response of the Pt-doped SnO 2 sensor is 33.12 at 300 ∘ C, while only 11.15, 13.42, and 16.89 for pure, Zn-, and Fe-doped sensors at 350 ∘ C.
Figure 5 represents the gas responses of the pure and Pt-, Fe-, and Zn-doped SnO 2 sensors as a function of CO gas concentration with sensor working at its own optimum operating temperature as measured above.As shown in Figure 5, each sensor exhibits a nearly linear gas response curve against CO with gas concentration in the range of 5-300 ppm.The Pt-doped SnO 2 sensor exhibits enhanced CO sensing response compared with pure, Fe-, and Zn-doped sensors, which implies Pt doping can effectively improve the gas response of SnO 2 sensor to CO gas.Thus, the SnO 2 nanosphere doped with Pt 2+ is a promising CO sensing material for fabricating high response CO sensors.
It is well known that response and recovery characteristics are important for evaluating the performances of semiconductor oxide sensors.Figure 6 shows the response and recovery properties of the Pt-, Fe-, and Zn-doped SnO 2 nanospheres sensors to 100 ppm of CO gas at each optimum operating temperature.It can be seen in Figure 6 that compared with Fe-and Zn-doped SnO 2 nanospheres, the sensor based on 2 at% Pt-decorated SnO 2 nanospheres shows shortest response and recovery time (8 s and 12 s, resp.).Such a rapid response and recovery property could be attributed to the excellent electronic sensitization and catalytic activities of Pt ions.
The gas responses of the sensors to 200 ppm of CO gas were investigated in two months to investigate its stability and repeatability.The measurements were reported every 10 days.As shown in Figure 7, slight variation is observed for Ptdoped sensor during the long experimental cycle, while much higher fluctuations are performed for pure, Fe-, and Zndoped sensors.All results demonstrate the Pt-doped sensor exhibits more prominent long-term stability and repeatability against CO than pure, Fe-, and Zn-doped SnO 2 nanospheres sensors.
Selectivity is another important characteristic for metal oxide semiconductor sensors, especially with potential interference gases.Figure 8 depicts the histogram of the gas responses of Pt-, Fe-, and Zn-doped SnO 2 sensors to 100 ppm of CO and potential interference gases, including CH 3 OH, NH 3 , H 2 S, and NO.As shown in Figure 8, the sensor based on 2 at% Pt-doped SnO 2 nanospheres exhibits considerably lower response values to other potential interface gases at 300 ∘ C. All gas sensing properties measured above demonstrate the 2 at% Pt-doped SnO 2 nanospheres sensor particularly interesting for CO detection in practice.Based on the gas measurements above, we can conclude that Pt doping effectively enhances the gas sensing performances of SnO 2based sensor to CO.

Theoretical Calculations.
As a typical n-type semiconductor material, the gas sensing properties of SnO 2 are dominantly controlled by the surface.It is well known that the (110) surface is the most thermodynamically stable plane in tetragonal rutile SnO 2 [30,31], and it has been widely used to investigate SnO 2 surface properties, such as structural, electronic, optical, magnetic, and gas sensing properties.Therefore, we performed a first principles calculation to go insight into how the doped metallic ions influence the gas sensing performances of SnO 2 .Figure 9(a) shows an established 2 × 1 × 1 slab model of SnO 2 (110) surface, which is constituted of Sn 12 O 24 with nine layers.To preserve symmetry, the top and bottom layers of the slab were taken to be identical.The central five layers were constrained at their bulk-like sites, and the surface and subsurface layers on either side of the slab were allowed to relax freely in all directions.A surface vacuum slab of 10 Å was added to avoid unnecessary interactions between the slabs, and then a 4 × 2 × 1 supercell of Sn 48 O 96 was employed.
One Sn atom was first substituted by Pt, Fe, or Zn atom in the surface or subsurface layers to obtain the doping model (Sn 47 MO 96 ) with a doping ratio of 2.08%.Four probable doping configurations were illustrated as shown in Figure 9(b): substitution in Sn 5C , Sn 6C , sub-Sn 5C and sub-Sn 6C , cite, respectively.The doping formation energy   was defined as [34]   =  (Sn 47 MO 96 ) −  (Sn where  (Sn 47 MO 96 ) and  (Sn 48 O 96 ) represent the total energy of M-doped and pure SnO 2 supercells after geometry optimization, and  Sn and  M are the chemical potentials of each Sn and M atom in the bulk Sn and M crystal, respectively.According to the definition, the lower the doping formation energy   is, the more stability the doping model has.
As shown in Table 1, for each metal doping model, the doping formation energy of substitution in Sn 5C position is lower than that in Sn 6C , sub-Sn 5C , and sub-Sn 6C cites, respectively.Therefore, Pt, Fe, and Zn atoms are all prior to occupy the Sn 5C site on the top layer of SnO 2 (110) surface.In the following discussion, we mainly consider Pt, Fe, and Zn atoms substituted on Sn 5C site.
Figure 10 shows the calculated total density of states (DOS) of pure, Pt-, Fe-, and Zn-doped SnO 2 (110) surfaces.Compared with pure SnO 2 , the DOS of M-doped SnO 2 (110) surface shifted downward, its band gap narrowed, and some new doping levels appeared near the Femi level, which could benefit charge transfer between SnO 2 valence band and  As reported in former works, CO gas molecule prefers to be adsorbed on Sn 5C site with C end [7]; thus, in this study CO gas molecule was perpendicularly imported onto the Sn 5C atom with C end, and the initial vertical height between C atom of CO and Sn 5C atom of SnO 2 (110) surface was set as 2 Å, where C atom could bond with Sn 5C atom freely.
Figure 11 shows the partial density of states (PDOS) of CO gas molecule after adsorption.One can clearly see in Figure 11 that the PDOS of CO has a leftward moving trend and the moving range decreases in the order of Pt-, Fe-, Zn-doped and pure SnO 2 surfaces.As shown in (a), when CO was adsorbed on pure surface, almost no other changes were observed except a little of band shift.For Pt-doped one (b), the PDOS of CO assigned by 5 orbits nearly vanishes totally, and the 4 orbits also diminish mostly, which implies a strong chemical reaction and banding between C atom and Sn 5C atom.When CO was adsorbed on Fe-doped (c) and Zndoped SnO 2 models, similar changes are observed but much slightly.
Further quantitative understanding can be obtained by analyzing the charge transfer between CO molecule and adsorption surface [34].Based on the Mulliken population analysis, the corresponding change transfers are 0.06, 0.22, 0.10, and 0.08 e when CO molecule adsorbed on pure, Pt-, Fe-, and Zn-doped SnO 2 (110) surface.Electrons lost from CO are received by the SnO 2 surface, decreasing the height of barrier in the depletion region of SnO 2 and increasing the concentration of charge carriers.Thus, increasing output voltage and gas response were observed in the gas sensing analysis system.

Conclusions
In conclusion, a simple hydrothermal synthesis of pure, 2 at% Pt-, Fe-, and Zn-doped SnO 2 nanospheres was demonstrated in the present paper.The as-synthesized nanostructures were characterized by XRD, EDS, and FESEM.Chemical gas sensors were fabricated, and CO gas sensing properties were discussed in detail.Comparative gas sensing tests show that the 2 at% Pt-doped SnO 2 nanospheres sensor exhibits excellent CO sensing performance in terms of low operating temperature, high response, rapid response and recovery time, and good reproducibility.Moreover, it shows good selectivity to potential interference gases.Based on the first principles calculation, its sensing mechanism was discussed.All results demonstrate that the Pt-doped SnO 2 nanospheres may be a potential candidate for fabricating high-performance CO gas sensors.

2 Figure 4 : 2 Figure 5 : 2 Fe-doped SnO 2
Figure 4: Gas responses of the sensors to 100 ppm of CO at various operating temperatures.

Figure 6 :
Figure 6: Response and recovery curves of the sensors to 100 ppm of CO.

Figure 7 : 2 Fe-doped SnO 2
Figure 7: Stability and repeatability of the sensors to 100 ppm of CO.

Table 1 :
The doping formation energy of Sn 47 MO 96 .